LONDON – British soldiers began withdrawing yesterday from their last base in the southern Iraqi city of Basra, paving the way for fresh troop cuts and fueling worries about the security of the country's second-largest city and the surrounding region.

U.S. and Iraqi authorities have expressed concern that a broader British drawdown could jeopardize the region's rich oil resources and the land supply line from Kuwait to Baghdad and beyond. Some analysts also fear that British withdrawal could exacerbate a violent power struggle between rival Shiite groups in the sect's southern heartland.

Around 550 soldiers were leaving the downtown Basra Palace, one of deposed President Saddam Hussein's former compounds, to join 5,000 personnel at an air base 7 miles away on the fringes of the city. Defense officials said the withdrawal was going well but could take days to complete.

The Iraqi military sent hundreds of reinforcements to the city to prevent Shiite militias and criminal gangs from expanding their influence once the British have gone.

Former Prime Minister Tony Blair reduced the number of British troops in Iraq from 7,000 to 5,500 in February and left open the option of pulling out about 500 more personnel once Basra Palace was handed back to Iraqis.

Prime Minister Gordon Brown is due to set out future strategy for British operations in Iraq in a speech to parliament next month.

The British defense ministry said forces operating from Basra Air Station will “retain security responsibility for Basra until we hand it over to provincial Iraqi control, which we anticipate in the autumn.”

Basra Palace was to house British soldiers and diplomatic staff after the 2003 U.S.-led invasion and has come under daily mortar and rocket attack in recent months.

“This is a thoroughly sensible military decision,” said opposition Conservative lawmaker Patrick Mercer. “It will allow more troops to be withdrawn from Iraq in the autumn, just as Britain increases its numbers of troops in Afghanistan.”

In Basra, Major Mike Shearer, Britain's military spokesman, told reporters: “I can confirm that an operation is ongoing, but we will not give any further details.”

U.S. officials have raised concerns about the prospect of British troops leaving the city, which fell under the influence of Shiite religious parties and militias, some with ties to Iran, after the January 2005 election that brought Prime Minister Nouri al-Maliki to power.

The police force is heavily infiltrated by militias. Political rivals, liquor dealers, DVD shop owners and anyone who violated different groups' interpretations of Islam were subject to assassination by death squads.

Retired U.S. Army Gen. Jack Keane, who was vice chief of staff at the time the Iraq war was launched, said in an interview last week that Britain had never deployed enough troops to properly stabilize the region and allowed a bad security situation deteriorate.

“It has always been our intention to draw down troops in Basra” as Iraqi army and police become ready to handle security duties, said a spokesman for Brown's Downing Street office, speaking on condition of anonymity.